Field of the Invention
This invention relates to a compound of an intumescent powder and a thermoplastic material and to the method of making the compound.
Intumescent powders when heated above their temperatures of intumescence swell to several or many times their volume. In accord with this invention they are mixed with a thermoplastic material to form a compound which may be molded to form articles which, in case of fire, swell to close apertures, floor or wall passages, wall spaces, door fittings, containers, anchor loose products or the like, to lower the chances of fire damage.
An intumescent powder contained in a thermoplastic matrix is of considerable use for the provision of articles for such uses as those recited above in potential fire vulnerable areas where the intumescent-thermoplastic compound will lessen the risk of fire, or fire damage or fire
Customarily the thermoplastic material is heated to a plastic state for mixing with an intumescent powder. Such mixing must take place at a temperature below the intumescence temperature of the powder to avoid premature intumescence. Thus the only thermoplastics which can be useful for mixture with the powder are those whose plastic state occurs at temperature lower than the powder""s intumescence temperature. By xe2x80x98plasticxe2x80x99 or xe2x80x98plastic statexe2x80x99 herein we mean in a condition capable of being molded or formed.
Heretofore the intumescent powders available had an intumescent point of 120xc2x0 C. to 140xc2x0 C. This limited the thermoplastic materials to which could be combined with prior art powders typically room temperature curing materials such as: rubber, latex, silicone and other materials which cure below 120xc2x0 C.
We have developed an intumescent powder with an intumescent point of about 195xc2x0 C. to 220xc2x0 C. As a result thermoplastic materials which are plastic below 195xc2x0 C. are now available for mixing (referred to herein as xe2x80x9ccompoundingxe2x80x9d because of the high viscosity of the flowable thermoplastic) and are very suitable for making the products, such as those recited above which reduce fire vulnerability. The intumescent powder is preferably 5 to 50% by weight of the mixture of powder, on the one hand, and thermoplastic and any filler on the other. The percentage of powder may be used to determine the swelling of a molded article in a fire.
The product of the compounding may be pelletized or chipped and then processed by extrusion injection molding, or other process to form the articles discussed.
The novel intumescent powder comprises alkali silicate powder containing a relatively significant amount of lithium oxide; the alkali silicate having a high degree of intumescence and a temperature of intumescence above the plastic temperature of thermoplastic materials such as those listed above.
The alkali silicate powder is a mixed Li/Na/K silicate. (For use in the inventive powder sodium and potassium are almost equivalent on a mole to mole basis so that it is not thought necessary in connection with this invention to specify them separately but merely to specify the content by molecular fraction of the sum of the sodium and potassium molar values relative to the content of the other ingredients.)
Two main factors influence the characteristics of the mixed Li:Na:K silicates of the alkali silicate powder. One is the silica to cation oxide molar ratio which can be expressed as the ratio y SiO2.M2O where y is part of the numerator of the ratio when the denominator is 1.00; and where M2O (equal to 1.00) is the total metal oxide which may include a Li2O, b Na2O, and c K2O when a, b, c are the relative molar quantities of the cation oxides and total 1.00. The molar ratios herein are considered to be accurate to plus or minus 2%.
The other is the ratio of cation oxide a Li2O: b Na2O: c K2O. The relative quantities of a Li2O, b Na2O, and c K2O total units as the denominator (M2O=1.00) for the ratio SiO2:M2O in the ratio described.
In view of the similarity in effects between sodium and potassium; the molar ratioxe2x80x94a Li2O, b Na2O, and c K2O may be written as: a Li2O: d (Na2O+K2O) (where d is the sum of b and c in moles) and a+d=1.00.
For brevity it is sometimes herein desired to indicate the mixture numerically only. For example a ratio 3.00/0.31:0.33:0.35 shall be read herein to indicate 3.00 SiO2/0.31Li2O:0.33 Na2:0.35 K2O and the ratio 3.00/0.31:0.69 shall be interpreted to indicate the molar ratio 3.00 SiO2/0.31 LiO2:0.69 (Na2O+K2O).
The tests conducted on the water solubility of ternary Li/Na/K silicates indicate that potassium and sodium silicates are similar in solubility.
The formulations of the intumescent powder within the scope of the invention range over the molar ratios (2.20 to 3.70) SiO2/(0.20-0.35)Li2O:(0.80 to 0.65)(Na2O+K2O) where the coefficients of Li2O and (Na2O+K2O) are chosen so that their sum equals 1.00 These are dried to a moisture content of 8-12% by weight and preferably ground to 50 to 500 microns.)
The formulation ratios are accurate to about 2 parts in 100 which is the commercial standards of accuracy for the materials specified and constitutes the tolerated variation in the ratios as expressed in the specification and claims.
The preferred limits of the formulations may be commented on as follows. For values of SiO2 below 2.20 the temperature of intumescence is lower than desired. Also the product tends to be less water resistant, and more readily efflorescent and more hygroscopic. For values of SiO2 above 3.70 the degree of intumescence is less than desired although there is good water resistance, low efflorescence and less hygroscopic effect.
For values of Li2O less than 0.20 it should be realized that substantially all the water remaining after drying (discussed hereafter) is linked to the Li2O, thus the decrease in the Li2O to less than 0.20 contemplated above is linked to a smaller amount of water which may be insufficient to cause a desirable degree of intumescence.
For values of Li2O greater than 0.35 the degree of intumescence is reduced below the limits desired.
For use with the newly developed thermoplastic powder, thermoplastic materials such as: polyvinyl chloride (PVC), polyethylene and polypropylene have been most commonly used. Examples of other thermoplastic materials which can be compounded with the novel powder include: ethyl vinyl acetate (EVA), acrylonitrile butadiene styrene (ABS) copolymers, polyethylene acrylates, and thermoplastic polyester. There are other thermoplastic materials with which the powder may be combined.
Of the thermoplastic materials we prefer to use PVC. When the PVC is heated to combustion temperatures chlorine is released which acts as a flame suppressant. Moreover the PVC does not completely burn and lends coherence to the char caused by intumescence.
The compounded powder and thermoplastic material may be mixed with a filler. The filler may be used to save expense of thermoplastic and clay may be used for this purpose. In other applications we prefer to use as filler alumina trihydrate since it is endothermic, releasing moisture when heated to combustion temperature, and hence the alumina trihydrate adds to the fire retardant properties of the compounded material.
The intumescent plastic compound is made by grinding the dried intumescent powder to the desired size. Ranges of 50 to 500 microns have been considered. However at sizes over 500 microns the compound containing the powder is difficult to mold. At sizes below 150 microns the intumescence is reduced. A preferred compound is (Sample xe2x80x98(B)xe2x80x99 hereafter) with a formulation 2.48 SiO2/0.26 Li2O:0.74(Na2O+K2O). Also with increased grinding the percentage of xe2x80x98finesxe2x80x99 (about xe2x88x92140 mesh 106 microns) becomes proportionately high and the fines tend to clog the separating screens. Thus we prefer to use powder of minus 35 mesh (less than 500 microns) which has an average size of 325 microns. At average size of about 325 microns the degree of intumescence is 80-110 (depending on the heating rate and water content) In spite of the objections to fine powder expressed herein we have employed powders of as low as 20 microns for coating fine wire.